IGN says Yoshio Sakamoto (Metroid, WarioWare, Tomodachi Life, etc.) and his team were behind Miitomo's development. Nintendo declines to talk about Sakamoto's team size or how long Miitomo spent in development.

This pretty much further busts the Tanabe/Retro Studios fallout rumor. Sakamoto couldn't be at Retro Studios since he was busy with Miitomo's development.

EDIT - Miitomo has passed one million downloads on iOS and Android combined in Japan in just three days:

Well...NoA put the March 2016 release date back on the English Miitomo website. Unless they really think they can get Miitomo and My Nintendo out before Friday (which would be April Fools'), I really don't expect to see either of them this week but rather later at e3 2016 instead.

Twitter is probably the easiest thing for most, since it's quick to setup and simple to just do quick follows, so that's probably the best approach overall. If you don't have one, just setup a bullshit account real quick and you're set.